Conventionally, in the field of blood drawing, a blood component such as platelets is collected by collecting only the component from drawn blood and returning the remaining blood components to the blood donor. In such operation, a blood component separation device including a centrifugal separator is used.
In recent years, in the field of radiation therapy of cancer or the like, transfusion of platelet liquid is widely performed, and high-concentration platelet liquid is necessary. To obtain high-concentration platelet liquid, Patent Literature 1 discloses an art using a blood component separation device to temporarily store low-concentration platelet liquid in a buffy coat bag and store only high-concentration platelet liquid in a platelet intermediate bag. Thus, from the centrifugal separator, the low-concentration platelet liquid flows out first, then the high-concentration platelet liquid flows out, and finally the low-concentration platelet liquid flows out. When the low-concentration platelet liquid, which flows out first and last, is stored in the platelet intermediate bag, the concentration of the platelet liquid stored in the platelet intermediate bag inevitably decreases. To prevent the decrease in concentration, the low-concentration platelet liquid that flows out first and last is temporarily stored in the buffy coat bag. Then in the second cycle, the temporarily stored low-concentration platelet liquid is mixed with whole blood drawn from the blood donor and transferred to the centrifugal separator. By repeating this process, only high-concentration platelet liquid is stored in the platelet intermediate bag.